So you need an op amp…and you can’t use just any generic op amp because of certain requirements. For example, a low supply voltage (<10V), high frequency, low noise, low power requirements etc.
Finding the right op amp isn't hard, here is an example of how to go about your search. For this example, I will specify the op amp to be used for the circuit from last week, a tape-head preamplifier. The schematic used an OP37. However, this op amp requires a 8V powersupply, and I'm going to be using only a 3V battery. Also, its good to see if there are any cheaper alternatives.
Specifically, the op amp is going to amplify a 10uV peak-to-peak signal by 1000 (up to 10mVpp). The op amp should work from 20Hz to 20KHz (optimal audio range), & be not too tough to solder.
First, write down what the constraints are:
- Has to run on a 3V “single supply” and we’ll have our DC offset at 1.5V
- Input offset voltage (Vos) must be < 1mV. At worst, at x1000 thats a +-1V offset at the output: as low as .5V or as high as 2.5V.
- Since we need to handle such a large swing, it should be rail-to-rail to within .2V of the rails to give us space.
- 1000 gain at 20KHz means the gain bandwidth (GBW) must be > 20MHz (in reality it can be much lower because the tape probably isnt good enough to record past 12KHz)
- Since our input signal is 10uV then (at worst) we want to have less than 5% noise (which isn’t so good but we’re willing to have poor quality audio). 5% of 10uV = 500nV. 500nV/sqrt(20000) = 3.5nVrtHz noise figure.
- Package should be SOIC for easy soldering
- Low power is nice but not -that- necessary, maybe < 10mA
- Low price! Must be under $2 at quantity 100, the lower the better
Stuff we don’t care about:
- Operating temperature
- Slew rate (at 10mV and 20KHz, it can be really slow)
The OP37, for comparison, has 12MHz GBW, requires 8V power supply, not rail-to-rail, Vos = 0.03mV, and noise figure of 3.2nVrtHz. So: great offset, great noise, so-so bandwidth and incompatible power requirements.
Lets go to TI and see what they have to offer. Select >=16MHz GBW, rail-to-rail and 8-SOIC package. The only chip they’ve got is the OPA350.
Now lets try National. Their system is a little tougher to use: click on 1mA offset first, which will pare down the options to 5 items, none of which are SOIC. You’ll notice national doesnt have a very good selection of low noise, ~20MHz op amps.
Next, we go to Analog Devices. Enter in package = SOIC, Vcc-Vee = 3V, noise <= 5nVrtHz, Vos <= 1mV then sort by price. The AD8655 looks good, a little noiser but nearly half the price.
Checking STMicro (click on “low noise”) they don’t have anything that runs on 3V.
Linear Technologies has a bunch of incredibly low noise op amps, at extraordinary bandwidths, but they’re rather expensive, at least $3 which is outside of our budget.
Maxim has a few good options (sort by noise, then compare all of the ones <5nVrtHz, then select out the ones that cost more than $2, then select only the ones that work from a single 2.7V supply & single package & > 20MHz GBW, that leaves the MAX4488.
| Part |
Noise (nVrtHz) |
GBW (MHz) |
Vos (mV) |
Iq (mA) |
$/qty 100
|
| OP37 |
3.2 |
12 |
0.03 |
5.5 |
$1.07 |
| OPA350 |
5 |
38 |
0.5 |
7.5 |
$1.73 |
| AD8655 |
4 |
29 |
0.05 |
4.5 |
$0.88 |
| MAX4488 |
4.5 |
42 |
0.75 |
2.5 |
$0.72 |
There are a couple other manufacturers but we’ve covered the most common. So let’s stop here. The OPA350 is clearly not a good choice, it’s the most expensive, noisiest and most power-hungry. So we’ll just ignore that, leaving the AD8655 and MAX4488. Now its just a comparison between price, power and noise. Since our noise figure is already higher than we’d like, I’ll place priority on that: the price difference is pretty small and power isn’t a huge priority.
Finally, we have chosen the AD8655. Yay Analog Devices!
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